Static Variables and Threads (C)
Your assumption (threads have their own copy) is not correct. The main problem with code is when multiple threads call that function get_uid(), there's a possible race condition as to which threads increments i and gets the ID which may not be unique.
All the threads of a process share the same address space. Since i is a static variable, it has a fixed address. Its "state" is just the content of the memory at that address, which is shared by all the threads.
The postfix ++ operator increments its argument and yields the value of the argument before the increment. The order in which these are done is not defined. One possible implementation is
copy i to R1copy R1 to R2increment R2copy R2 to ireturn R1If more than one thread is running, they can both be executing these instructions simultaneously or interspersed. Work out for yourself sequences where various results obtain. (Note that each thread does have its own register state, even for threads running on the same CPU, because registers are saved and restored when threads are switched.)
A situation like this where there are different results depending on the indeterministic ordering of operations in different threads is called a race condition, because there's a "race" among the different threads as to which one does which operation first.
No, if you want a variable which value depends upon the thread in which it is used, you should have a look at Thread Local Storage.
A static variable, you can imagine it really like a completely global variable. It's really much the same. So it's shared by the whole system that knows its address.
EDIT: also as a comment reminds it, if you keep this implementation as a static variable, race conditions could make that the value i is incremented at the same time by several threads, meaning that you don't have any idea of the value which will be returned by the function calls. In such cases, you should protect access by so called synchronization objects like mutexes or critical sections.